Glutamate is the major excitatory neurotransmitter in the central nervous system. Three glutamate receptor ion channel subtypes have been identified based on their sensitivity to the selective activators (agonists) N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate.
AMPA receptors mediate cellular responses to glutamate by direct and indirect mechanisms. When activated by glutamate or AMPA, AMPA receptor ion channels allow sodium ions (Na+) and calcium ions (Ca2+) to pass directly through the channel pore. In addition, AMPA receptor ion channels can facilitate the activation of NMDA receptors by initiating cellular depolarization that relieves magnesium ion (Mg2+)-dependent block of NMDA receptors.
Multiple AMPA receptor subtypes have been identified and cloned: GluR1, GluR2, GluR3, and GluR4 as disclosed by Hollmann and Heinemann, Ann. Rev. Neurosci., 17, 31-108 (1994). Each subunit consists of a sequence of approximately 900 amino acids. Four subunits are thought to assemble to form a tetrameric ion channel complex with the functional properties of this ion channel most likely being determined by its subunit composition.
Ion channel currents activated by glutamate via AMPA receptors are transient. The time course of currents is modified by refractory states caused during glutamate binding which is referred to as desensitization and by the rate of glutamate removal from the ion channel binding site which results in deactivation. Ion influx through AMPA receptors may be enhanced by compounds that either prevent desensitization or by compounds that slow deactivation rates. Compounds that enhance glutamate-stimulated ion influx at AMPA receptors are known as positive AMPA receptor allosteric modulators or AMPA receptor potentiators. One such compound, which selectively potentiates AMPA receptor function, is cyclothiazide. Since AMPA receptors play a pivotal role in mediating fast excitatory transmission in the central nervous system, molecules that enhance AMPA receptor function have multiple therapeutic targets.
Compounds that allosterically potentiate AMPA receptors have been shown to enhance synaptic activity in vitro and in vivo as disclosed, for example, by I. Ito, et al., J. Physiol., 424, 533-543 (1990) and A. Copani, et al., Journal of Neurochemistry, 58, 1199-1204(1992). Such compounds have also been shown to enhance learning and memory in rats, monkeys, and humans, and are reviewed by Gouliaev and Senning, Brain Research Reviews, 19, 180-222 (1994).
International Patent Application Publication WO 98/33496 published Aug. 6, 1998 discloses certain sulfonamide derivatives which are useful, for example, for treating psychiatric and neurological disorders, for example cognitive disorders, Alzheimer's disease, age-related dementias, age-induced memory impairment, tardive dyskinesia, Huntington's chorea, myoclonus, Parkinson's disease, reversal of drug-induced states (such as cocaine, amphetamines, alcohol-induced states), depression, attention deficit disorder, attention deficit hyperactivity disorder, psychosis, cognitive deficits associated with psychosis, and drug-induced psychosis. P. L. Ornstein, et al. J. Med. Chem., 43, 4354(2000) further disclose biarylpropylsulfonamides which are potent potentiators of AMPA receptors. In addition, X. Li, et al., Neuropharmacology, 40, 1028 (2001) disclose antidepressant-like actions of an AMPA receptor potentiators. D. D. Schoepp, et al. and Tizzano, et al., Society for Neuroscience Abstracts, 26(1-2), 528.19 and 528.20, 30th Annual Meeting, New Orleans, (Nov. 4-9, 2000) disclose an orally active AMPA receptor potentiator that enhances spatial learning and memory performance in rats, and reverses both pharmacologically and age-associated learning and memory deficit in rats. New AMPA receptor potentiators are needed to treat these neurological disorders.